How to handle images in text widget withoug logging in?

Here’s my problem. I want show in my blog sidebar some affiliate ads. One affiliate had a banner ad with the HTML to place the image from their site to my sidebar and complete my hop link connection. But then I wanted to re-size the banner but I couldn’t use their HTML because it linked to their original fixed banner size.

I thought the best thing was to re-size the image and put it on my host in the public directory. That gave me an FTP address for my image URL, but when I ran the image location HTML in my WordPress text widget, it asked for the login and password to my host.

So, what is the best way to keep control of and use my re-sizd banner ads so folks don’t need to login to see them?

Thanks!